The present invention relates to a process for producing a polyimide platy object, which can be used as a substrate of optical devices, electronic devices or electrooptical devices, and relates to such polyimide platy object having a sufficient strength and a sufficient surface flatness.
Materials for optical waveguides are required to have characteristics, for example, that optical loss is small, that their production procedures are simple, that their core and cladding can be well controlled in refractive index, and that they are improved in heat resistance. Hitherto, quartz-based materials have been studied the most intensively among optical waveguide materials. Quartz-based materials, however, have some defects in their production that they require a long time for producing an optical waveguide, that they require a heating at high temperature, and that it is difficult to form them into a piece of large area. In view of this, polymer materials (e.g., polymethylmethacrylate, polycarbonate, polysiloxane, and polyimide) have attracted much attention in recent years as hopeful optical waveguide materials.
In fact, polymethylmethacrylate has an advantage that it is possible to form an optical waveguide at low temperature with low production cost. In contrast, it is inferior in heat resistance and humidity resistance.
Polyimide is considerably superior to other plastics in heat resistance. Conventional polyimides were, however, inferior in light transmission. To overcome this defect, some studies have been conducted. Japanese Patent Unexamined Publication JP-A-3-72528 discloses a fluorinated polyimide superior in light transmission. JP-A-4-8734 discloses a fluorinated polyimide copolymer capable of controlling refractive index for the formation of an optical waveguide. JP-A-4-9807, JP-A-4-235505 and JP-A-4-235506 disclose fluorinated polyimide optical waveguides.
Even such polyimide optical waveguides, which are superior in light transmission and heat resistance, have some defects. For example, it is a problem of birefringence (double refraction). The occurrence of birefringence in polyimide optical waveguides is caused by residual stress remaining in the inside of the waveguides. It is known that this residual stress is caused by the difference in thermal expansion coefficient between polyimide and its substrate. In connection with this, JP-A-9-15608 discloses a polyimide film that is low in birefringence, produced by using a polyimide substrate for supporting thereon the polyimide film. With this, it becomes possible to eliminate the above-mentioned problem of thermal expansion coefficient difference therebetween. Such polyimide substrates are, however, generally large in surface roughness. Therefore, polyimide optical waveguides formed on such polyimide substrates have a defect that they do not easily transmit light therethrough, causing a large optical loss. Thus, such polyimide optical waveguides are not suitable for optical use. In connection with this, JP-A-11-23870 discloses that it is necessary to grind the surface of conventional polyimide substrates in order to use them for optical use and that an easily grindable film is formed on a polyimide substrate, followed by grinding of this film.
JP-A-11-262985 discloses a process for producing a polyimide substrate by laminating polyimide films using an adhesive (e.g., acrylic resin adhesives, epoxy resin adhesives). The use of such adhesive causes damage to the original characteristics of polyimide. That is, its heat resistance becomes inferior, and its thermal expansion coefficient deviates from the original value. There are some other problems, too. For example, it is necessary to have a complicated facility for laminating polyimide films in an industrial scale. Furthermore, it is difficult to adjust the thickness of such polyimide substrate.
Each of JP-A-10-45918 and JP-A-11-302380 discloses a process for molding a polyimide powder into a molded product. Since the particle size of the polyimide powder has an influence on the surface roughness of the molded product, it is difficult to achieve a sufficient flatness of the molded product. Therefore, it is necessary to conduct a grinding or the like to obtain its flatness.
It is therefore an object of the present invention to provide a polyimide platy object that is substantially free of the above-mentioned problem(s).
It is a more specific object of the present invention to provide a polyimide platy object that has superior heat resistance and superior strength of the original polyimide itself and a surface flatness sufficient for use in producing optical waveguides.
According to the present invention, there is provided a process for producing a polyimide platy object. This process comprises:
(a) providing a solution containing therein a solvent and a solute that is a polyimide precursor or a polyimide;
(b) pouring said solution onto a supporting member;
(c) removing a portion of said solvent from said solution by evaporation of said solvent at a first temperature that is lower than a boiling point of said solvent, thereby forming a precursory platy object on said supporting member,
said precursory platy object having a self-supporting property,
said precursory platy object having a first surface that is in contact with said supporting member and a second surface that is away from said supporting member,
said precursory platy object having a first content of said solvent at said first surface and a second content of said solvent at said second surface,
said first content being greater than said second content by an amount of 2-10 wt %;
(d) detaching said precursory platy object from said supporting member; and
(e) heating said precursory platy object at a second temperature that is higher than said boiling point of said solvent and is lower than a decomposition temperature of said polyimide, thereby producing said polyimide platy object.